Freitag, 9. August 2013

Human populations and the need for novelty and change:

>Just as your experience can influence your innate temperamental inclinations, culture can shape a population's genetic makeup and thus its mores. Conventional wisdom has it that history is driven by social developments, from new technology like the wheel to new political ideas such as democracy. Proponents of the emerging field of biohistory, however, also see nature's hand in the unfolding human narrative. One of their most interesting arguments focuses on the neophiliac 7R allele's [an allele of the dopamine D4 receptor] radically uneven global distribution.

Research conducted at the University of California at Irvine by Robert Moyzis and his collaborators offers a provocative explanation for why some populations around the world have very high incidence of 7R and others, a very low one. A study led by psychologist Chuansheng Chen first posited that the allele began as a "migration gene". Restlessness can be a big benefit in certain situations, and Moyzis, who takes the sudden-and-recent view of behavioral evolution, thinks that the mutation helped Homo sapiens survive and thrive by spurring our vast African exodus to distant parts unknown.

Everyone who carries the 7R gene today, wether a European urbanite, a sub-Saharan villager, or a South American Indian, has essentially the same version, and using standard statistical methods for analyzing DNA sequences, Moyzis determined that 7R probably arose 50 000 to 40 000 years ago, right around the time of the great migrations. Pointing out that the allele is relatively new and that other primates don't have it at all, he says, "To have maintained a significant incidence in the current population, it must have been selected for, because if a mutation have no benefit, evolution will get rid of it."

Studies of populations around the world show that the groups whose forebears stayed closest to our African home at the time of the great migrations have a high incidence of our ancestral DRD4-4 allele, whose strong affinity for dopamine translates into strong regulation of behavioral excitability. In contrast, the descendants of those who traveled the farthest have the greatest frequency of the genetic variants linked to a low affinity for dopamine and robust novelty seeking: 2R, 5R, and especially 7R. As much as 25 percent of the population in much of Europe, as well as their American descendants, carry 7R, but the highest incidence - up to 85 percent in some groups- occurs among the South American Indian tribes who live farthest from Africa in areas such as the Amazon basin.

At first, it's hard to believe the research that shows that the novelty-seeking 7R allele hardly exists in China anymore. Moyzis thinks that it mutated into the much shorter 2R version, which suggests a selection against the most neophiliac variant. The educated guess as to why and how such a genetic shift occurred in China is "cultural selection" by whatever means. (Offering a seemingly tongue-in-cheek example, University of Utah anthropologist and biohistorian Henry Harpending recalls that when he asked a Chinese biogeneticist why the allele had almost disappeared, "without hesitating, the researcher said, 'Oh, we killed all the 7R people.' ") In one scenario, the Asian adventurers who carried the restless allele might have crossed the Bering Strait into the Americas, where its incidence is high. In another proposed by the Moyzis group, as China's society settled into an economy in which many people spent most of the day cultivating rice, a restless mind and an appetite for novelty became drawbacks. Even two thousand years ago, China's mandarin system would have favored the individuals likeliest to get ahead in its bureaucracy, so that unlike his friskier brother, a dutiful son who rose up in the ranks might have acquired multiple wives and produced many offspring.

Whatever its roots in the ancient past, China's traditionally conservative society is very different from, say America's freewheeling, risk-tolerant, inventive culture. In November 2010, the Chinese government, keenly aware of the country's reputation for imitation rather than innovation, announced a program for dramatically increasing its low number of patents. By offering incentives such as cash bonuses and better housing, the leadership hopes to produce more creators of technology like iPad rather than just efficient manufacturers of them.

It may sound politically incorrect, but over the past few thousand years, cultures and their differences appear to have influenced human DNA, especially in Asia. As Harpending puts it, "When I first saw that research relating an interesting and normal behavioral phenotype to a gene difference, I thought myself, 'Pandora's box is open now.' " This phenomenon is all the more fascinating when you consider that although the two populations are now so diametrically different in both 7R's frequency and the agendas of their societies, the New World Indians are thought to be descendants of Asians who migrated there perhaps 12 000 years ago. The same hyper, distractible, risk-taking characteristics associated with the allele that poorly suit traditional Chinese culture can be highly adaptive in other settings, from the Amazon basin to Silicon Valley, where selection could work in 7R's favor.

The history of 7R in China prompts speculation about its future in the increasingly sedentary, screen-oriented, postindustrial West. In our desk-tethered world, it's already hard to believe that it wasn't so long ago in human history that most people, much less small children, weren't expected to spend their days sitting still and concentrating on mental tasks for hours on end. Unlike Amazon tribesmen and rural Ariaals, many young novelty seekers in our urbanized environments lack the legitimate outlets for their high spirits long provided by our species' ancient, traditional pursuits of exploration and hunting. If also poorly raised and immature, they can end up making trouble for themselves and others. As Harpending says, "There are badly behaved kids who will pick up a brick and throw it through a store window, even though they are sure to get caught. It's just a way of relieving their own boredom."...<